A novel SCN4A mutation causing myotonia ... - Semantic Scholar

© 1993 Oxford University Press

Human Molecular Genetics, 1993, Vol 2, No. 9

1349-1353

A novel SCN4A mutation causing myotonia aggravated by cold and potassium Roland Heine, Ursula Plka and Frank Lehmann-Horn* Department of Appfed Physiology, Untverrty of Wm, 089069 Ulm, Germany

Received July 14, 1993; Accepted July 16, 1993

INTRODUCTION Different mutations in SCN4A, the gene encoding the adult skeletal muscle sodium channel a-subunit have been shown to cause the human hereditary disorders hyperkalemic periodic paralysis and paramyotonia congcnita.1"8 As a consequence, the long-standing question as to whether these two diseases are directly related was finally settled: thus the term 'sodium channel disease'7 was introduced to encompass the different allelic syndromes caused by SCN4A gene mutations. According to this definition, muscle sodium channel diseases encompass: i) classical paramyotonia congenha (PC) as characterized by the key symptoms of paradoxical myotonia (muscle stiffness aggravated by exercise) and cold-induced muscle stiffness followed by weakness,9'10 and its variants such as: PC with myotonic signs present even in a warm environment; PC without weakness, not even during strenuous exertion of the cooled muscles; and PC with spontaneous and potassium-induced attacks of weakness; ii) hyperkalemic periodic paralysis (HyperPP) characterized by the symptoms of spontaneous and potassium-induced attacks of weakness11 and associated with or without myotonia or paramyotoni.12 The relationship between these diseases and another condition characterized by muscle stiffness responsive to acetazolamide,13 a potassium-lowering drug, has not been clarified. We recently classified a third group of muscle sodium channel diseases, namely sodium channel myotonia.8-14 This disease is characterized by a dominant mode of inheritance, and has as its

1

To whom correspondence should be •ddictted

key symptom muscle stiffness without muscle weakness. Moreover, in the families examined die stiffness was not substantially aggravated by cold. In some of them myotonia is mild and considerably variable from day to day (myotonia fluctuans15-8). In other families the myotonia is very severe and continually present (permanent myotonia8). The molecular biological findings of different defects in the . gene encoding the muscle sodium channel had been anticipated by electrophysiological studies on excised muscle specimens from such patients. Experiments on native muscle fiber segments performed with the three-microekctrode voltage clamp and, later, also with patch-clamp methods have revealed that the sodium currents underlying the repetitive firing of muscle fiber action potentials show a peculiar failure to inactivate properly.8>16~18 In the present study we investigate the genetic defect in a family with dominant myotonia for which an earlier electrophysiologic study had revealed abnormal sodium currents.19 Based on the dominant inheritance and the clinical signs we had then misdiagnosed the disease as myotonia congenita (Thomsen), a disease now known to be caused by a defect in the muscle chloride channel.20 Now we are able to show that the symptoms are caused by a previously unidentified mutation in SCN4A, and thus provide further evidence for the existence of sodium channel myotonia. Some of the data were presented at the 17th ENMC Workshop on 'Non-dystrophic myotonias and periodic paralyses1.7

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The single strand conformation polymorphism (SSCP) technique was used to screen genomlc DMA of a family with myotonia aggravated by cold, potassium loading and suxamethonlum, but without muscle weakness. An aberrant band was found In exon 24 of SCN4A, the gene encoding the adult skeletal muscle sodium channel a-subunK. DNA sequencing led to the detection of a G-to-A transition of cDNA nucleotlde 4765 predicting a substitution of methlonlne for vallne at position 1589 of the protein sequence. This amlno acid Is located within transmembrane segment S6 of channel repeat IV dose to the cytoplasmlc surface, a region which is supposed to act as acceptor of the Inactivatlon gate of the channel. Four lines of evidence Indicate that this mutation causes the disease: (I) the transition was only found for affected family members; (II) no mutations were found In all other SCN4A exons; (HI) the affected gene region Is conserved among various species; and (Iv) an .Increase In the number of non-lnactlvatlng sodium channels had been revealed In earlier electrophyslologlcal studies on an excised muscle specimen from the Index patient. In addition, the dose-by occurring substitution of vallne for methlonlne at position 1592 known to cause hyperkalemic periodic paralysis was deduced for six families with the myotonte, non-dystrophlc form of this disease.

1350 Human Molecular Genetics, 1993, Vol. 2, No. 9 RESULTS Abnormalities in the mobility of single-strand DNA fragments of the SCN4A transcript were found for several members of family SCM8. All abnormalities were detected in the region of exon 24 which encodes the transmembrane segment S6 of repeat IV. For the affected family members, the index patient and his affected daughter, a peculiar aberrant band (A in Figure 1) was discovered which was not present in the DNA of the parents, the brothers and the son of the index patient. A second aberrant

9 T A C A T 0

T T G A C C A C G

A T O A G O

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T T G

A T G

A A

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Figure 1. Poryacrylamide gels showing single stnnd conformatioral polymorphisms for the flagmen of exon 24 of the muscle sodium channel atvpimil pwtw (SGN4A) which encodes the i* M seEffient So of channel (baud A) and his repeat IV. Aberrant binds were found for the index i affected daughter (bands A, B), her non-affected mothe and seven] other nonaffected members (open symbols) of family SCM8 and a normal control (band B); no aberrant bands were detected for the index patient's parents (not shown since paternity and maternity were not tested without agreement). In me same SCN4A fragment, another abemnt band (Cl) different from A and B was detected in the afTfuiwl nwnlifjs of families with the uiyotuuic form of HyperPP: shown are the bands for index patients of families HyperPPl and HyperPP7. For the latter, family pedigree and polymorphisms of several affected and non-affected family members are shown in the lower panel. Here, the aberrant band (C2) is different from Cl since the PCR products were made smaller by digestion (for details see Methods section).

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A C C A T G

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A A

V«l hmn Mat Tyr OTC AAC ATO TAC CTC AAC ATO TAC v«l kan Mat Tyr

Figure 2. A. A comparison of the wild-type and the mutant SCN4A sequence of cDNA nudeoudet 4736 to 4779 is shownJO The C to T base exchange (arrows) found in the iittttntv DNA sequence of die index patient of family SCM8 corresponds to a G to A transition in die sense DNA sequence (not shown). B. Tbe mutation predicts a substitution of mrrhinntnr for valine at position 1389.


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band was found for the daughter (Figure 1, band B). This band was also found for her mother and several other non-affected family members and also for a normal control. In the same exon, an aberrant band different from the two former ones was detected in six out of 25 HyperPP families (two examples in Figure 1, band Q . No exon 24 abnormalities were detected for 76 normal controls. For the index patient and his daughter, no abnormal polymorphisms were detected in exons 1 —23. For some normal controls, aberrant bands were found in exons previously described and, in addition, in exons 3 and 10 (not shown). By sequencing the DNA eluted from the aberrant band (A) which was found for the index patient and his daughter, a G-toA transition at position 4765 of the adult skeletal muscle sodium channel a-subunit cDNA was discovered, predicting a substitution of methionine for valine-1589 (Figure 2); no mutation was found in exons 1 —23 of the genomic DNA from the index patient. The aberrant band (B) found for the index patient's daughter and her mother was caused by a C/A polymorphism at cDNA nucleotide 4536 which has no effect on the amino acid. For the affected members of the six families with HyperPP, a previously described A-to-G transition at position 4774 was detected, predicting the substitution of valine for methionine-1592 (Figure 1; Rojas et al. 1991). All non-affected family members revealed neither the valto-met nor the met-to-val substitution (data not shown). Valine-1589 as well as methionine-1592 are highly conserved in all sodium channels sequenced to date (Figure 3). The aberrant bands in exons 3 and 10 were caused by polymorphisms: 403 A/C (135 val/met) and 1570 G/A (524 gly/ser).

Human Molecular Genetics, 1993, VoL 2, No. 9 1351 DISCUSSION The novel mutation within the gene encoding the muscle sodium rhrnmri a subunit is present in tworelatedpatients withmyotonia markedly aggravated in the cold, after oral intake of potassium or administration of suxamethonhim. Muscle weakness did not occur, even under provocative test conditions that induce in co*^ffff^Ftju i^fl^f^Tycaftytifl pflty-^lfl- in tru* post,

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showing such symptoms were therefore not classified as having PC but a special form of 'myotonia congenha with coldsensitivity', by Becker12. Accordingly, this was the diagnosis when the family in question was first studied19. Meanwhile, it has been shown by molecular genetics that myotonia congenita is a chloride channel disease.20"22 On the other hand, die results of the present study corroborate our earlier postulate for the existence of a 'sodium channel myotonia' as third group of muscle

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S t S t S S S S

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Figure 3. A. Schematic of the sodtam channel a-subanit camming of four region of internal bomology, to-called repeat*, connected by intraceflnlar loop*. Each repeat contains aix hydropbobic Kg"-"'? (SI to S6), putative transmembrane helices. Between «>)(•••"«« S5 and S6 of each repeat, an iitnlintrj is found consisting of an extracellular part and a sequence which dipt into the membrane. Thrft^wmtTmjnwnhrmf. innpi «tr thrmght m fhrm ihr. Kmng nf me chmnd pore. The orifice on the ""•*"•""'«'• tide of the pore or ks mrroanding protein parti are wppoied to act a acceptor of the inacovation gate. B. Amino add tequeoce of trgmrnti